1. Field of the Invention
This invention relates to an ophthalmic apparatus, for example, an optometric visual target apparatus or an ocular refractivity measuring apparatus used in the practice of optometry in an ophthalmic hospital or the like.
2. Description of the Prior Art
A conventional optometric apparatus for measuring ocular refractivity or the like incorporates an optometric visual target device for optically generating a distant-sight visual target. As used in this document, the terms "distant sight visual target" and "visual target for distant sight" refer to a visual target that 1) appears to the viewer to be at an infinite distance from the viewer, 2) appears to the viewer to be at approximately 5-6 m from the viewer, or 3) is actually approximately 5-6 m or further from the viewer. In addition, as used in this document, the term "visual target for near sight" refers to a visual target that appears to the viewer to be approximately 20-30 cm from the viewer or is actually approximately 20-30 cm from the viewer. There have been known, for example, a device which provides a separate visual target for each eye to be tested, as disclosed in Japanese Utility Model Application Publication No. 59-9912 (1984), and a device which provides a visual target using an edge prism, as disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 60-171031 (1985).
The above-described conventional devices, however, have the following problems. In the device which provides a separate visual target for each eye, two visual targets must be provided within the device, and so a wide field of view cannot be obtained. In the device which uses a wedge prism, an aberration due to the wedge prism is produced, distorting an image. In the both devices, the natural visual sensation of the visual target experienced by the user is impaired, and so complete remissive adjustment cannot be performed. Hence, there is the possibility of inducing mechanical myopia, influencing the optometric value.
In a conventional subjective ocular refractivity measurement, a visual target for distant sight is disposed at a far distance of 5 m and a lens is mounted in front of an eye to be tested, and the diopter value of the lens is changed so that the visual target can be recognized. However, such an arrangement takes up a large amount of space. Therefore, various kinds of apparatuses have been devised for the purpose of saving space. These include an apparatus which provides an eye to be tested with a visual target actually disposed near the eye which appears as if the visual target is at a far distance by means of an optical system using a stereoscopic microscope; an apparatus which makes the optical path from a visual target disposed near an eye to be tested to the eye long by using an optical system combining a plurality of mirrors; and an apparatus which provides a collimating lens and a visual target in front of each eye to be tested, each visual target being disposed at the position of the focus of its collimating lens.
The above-described conventional apparatuses, however, have the following disadvantages. In the apparatus using a stereoscopic microscope, since a Maxwellian view is provided, part of the field of view is cut off if the face of the person to be tested moves. In the apparatus which uses an optical system combining a plurality of mirrors, since a large space is occupied by the mirrors within the apparatus, it is difficult to design a large field of view. In the apparatus which provides each eye to be tested with a collimating lens and a visual target, a large field of view cannot be provided. In any of the conventional apparatuses, part of the field of view frequently is cut off or is small during measurement, and therefore, natural visual sensation is not provided, and so complete remissive adjustment cannot be performed, inducing in some cases mechanical myopia. Accordingly, the measured value obtained by such an apparatus differs from the measured value obtained by an ocular refractivity measuring apparatus when a visual target disposed at a far distance of 5 m is actually provided, and hence accuracy in measurement by the conventional apparatus is low, preventing wide distribution of such apparatuses.
An apparatus which provides a visual target using a Maxwellian view has been disclosed, for example, in Japanese Patent Application Public Disclosure (kokai) No. 59-85641 (1984). This approach, however, has the disadvantage that a visual target cannot be observed if the position of an eye moves a little, and so a sufficiently wide field of view cannot be provided.